1. Field of the Invention
The present invention relates to an optical pickup compatible with optical recording media having respectively different specifications.
2. Description of the Related Art
There are discs, cards or tapes, etc., used as recording media for recording and reproducing information such as video, audio or data at high density. Among them, discs are widely being used. In the structure of one type of disc, a plastic or glass medium having a certain thickness exists along an axial direction through which light is incident, and an information recording surface exists on the medium. Recently, the optical disc field has been developed from a laser disc (LD) and a compact disc (CD) to a digital video disc (DVD).
At present, a high density optical disc system uses an objective lens having a large numerical aperture (NA) and a light source of short wavelength light of 635 nm or 650 nm, in order to enhance a recording density. The use of the short wavelength light source enables reproduction of data from a CD having a different thickness from that of a DVD, as well as recording and reproduction of information on/from the DVD. However, the system should use light of 780 nm wavelength in order to be compatible with a writable compact disc (CD-R) which is a recent type of CD. This is due to a recording characteristic of the CD-R recording medium. Thus, it has been crucial to develop an optical pickup which can use light of both 780 nm and 650 nm wavelengths, in order to compatibly use both a DVD and a CD-R. As a disc specification changes, an optical aberration occurs due to a change in disc thickness, a change of wavelength and a change of a numerical aperture. Accordingly, a search for an optical pickup compatible with different specifications while removing the optical aberration is actively being pursued. As a result of the search, optical pickups compatible with different specifications are being manufactured. An existing optical pickup compatible with a DVD and a CD-R will be described below with reference to FIG. 1.
FIG. 1 shows an existing optical pickup using a conventional variable iris. The optical pickup of FIG. 1 uses laser light of 635 nm wavelength during reproduction of a DVD and uses laser light of 780 nm wavelength during recording and reproduction of a CD-R. The light of 635 nm wavelength emitted from a light source 1 which is a laser diode is incident to a collimating lens 2. The light beams of 635 nm wavelength are shown as solid lines. The collimating lens 2 converts the incident light into parallel light. The light having passed through the collimating lens 2 is reflected by a polarizing beam splitter 3 and then proceeds to an interference filter prism 4.
Meanwhile, the light of 780 nm wavelength emitted from a light source 11 being a laser diode passes through a collimating lens 12, a beam splitter 13, and a focusing lens 14 and then proceeds to the interference filter prism 4. The light beams of 780 nm wavelength are shown as dotted lines. The interference filter prism 4 totally transmits the light of 635 nm wavelength reflected from the polarizing beam splitter 3 and totally reflects the light of 780 nm wavelength focused by the focusing lens 14. As a result, the light emitted from the light source 1 is incident to a wavelength plate 5 in the form of the parallel light made by the collimating lens 2. The light emitted from the light source 11 is incident to the wavelength plate 5 in the form of diverging light. The light transmitting through the wavelength plate 5 passes through a thin-film type variable iris 6 and then is incident to an objective lens 7.
The objective lens 7 is designed to focus the light of 635 nm wavelength on an information recording surface of a DVD 8 having a thickness of 0.6 mm. The light of 635 nm wavelength having passed through the variable iris 6 is focused on the information recording surface of the DVD 8. As a result, the light reflected from the information recording surface of the DVD 8 contains the information recorded on the information recording surface. The reflected light is transmitted back through the objective lens 7, the thin-film type variable iris 6, the wavelength plate 5, the interference filter prism 4 and the polarizing beam splitter 3 and is detected by a photo detector 10.
Also, the objective lens 7 focuses the light of 780 nm wavelength having passed through the variable iris 6 on the information recording surface of the CD-R 9 having a thickness of 1.2 mm. In this case, a spherical aberration occurs due to the difference of thickness between the DVD 8 and the CD-R 9. The spherical aberration is due to the fact that the information recording surface of the CD-R 9 is located at a farther place from the objective lens 7 along its optical axis than that of the DVD 8. When the variable iris 6 to be described later referring to FIG. 2 is used, the light of 780 nm wavelength forms an optical spot of a size optimized to the CD-R 9 on the information recording surface of the CD-R 9. The light of 780 nm wavelength reflected from the CD-R 9 is transmitted back through the objective lens 7, the thin-film type variable iris 6 and the wavelength plate 5, is reflected by the interference filter prism, transmitted through the focusing lens 14, reflected by the beam splitter 13 and detected by a photo detector 15.
As shown in FIG. 2, the variable iris 6 of FIG. 1 has a thin film type structure which can selectively transmit the light incident to the area of not more than NA 0.6 corresponding to the diameter of the objective lens 7. That is, the variable iris 6 includes a first area where the light of both 635 nm and 780 nm wavelengths are transmitted and a second area where the light of 635 nm wavelength is totally transmitted and the light of 780 nm wavelength is totally reflected. The first area is an area having a numerical aperture not more than 0.45 and the second area is an outer area of the first area. Also, the first area is formed of a quartz (SiO.sub.2) thin film in order to remove the optical aberration due to the second area formed of a dielectric thin film. By using the variable iris 6, the light of 780 nm wavelength passing through the first area of not more than NA 0.45 forms the optical spot appropriate for the CD-R 9 on the information recording surface thereof. As a result, the optical pickup of FIG. 1 is compatibly used with the optimized optical spot even when the optical recording medium is changed from the DVD 8 to the CD-R 9.
However, the optical pickup of FIG. 1 forms a finite optical system with respect to the light of 780 nm wavelength in order to remove the spherical aberration occurring when exchanging the DVD with the CD-R. For this reason, the structure of the optical system is complicated and the assembly of the optical components is difficult. In addition, since an optical path difference occurs between the light passing through the first area of not more than NA 0.45 and the second area of not less than NA 0.45, due to the dielectric thin film formed in the second area of not less than NA 0.45 of the variable iris 6, the first area needs to be formed of a special optical thin film (SiO.sub.2 thin film). Accordingly, the first area has been formed of an SiO.sub.2 thin film and the second area has been formed of a multi-layer thin film. However, since the manufacturing process is complicated and the adjustment of the thin film thickness should be done at high precision in units of ".mu.m", it is not ideal for mass production. Also, although some techniques, in which the wavelengths of the light to be used are varied and a single lens is used, have been known in the direct overwrite technique for performing recording and reproduction operations simultaneously, such techniques cannot perform recording and reproduction operations with respect to different discs having different specifications.